Solubility of TiO2 in BaTiO3

Scanning electron microscopy and electron probe micro-analysis were used to investigate the microstructure of both slow-cooled and quenched polycrystalline BaTiO₃ specimens with a small excess of TiO₂ (Ba/Ti=0.995 to 0.999) or of BaO (Ba/Ti=1.002 and 1.005). The electron micrographs of polished and etched TiO₂-excess BaTiOs samples, and of fracture surfaces of quenched samples, showed a second phase in the grain boundaries and triple-point regions, whereas no second phase was observed in samples having Ba/Ti=1.000. Microprobe analysis of the second phase gave compositions near that of the reported adjacent phase of higher TiO₂ content, Ba₆Ti₁₇O₄₀. The results indicate that the solubility of TiO₂ in BaTiO₃ is <0.1 mol%.     

{111} Twin Formation and Abnormal Grain Growth in Barium Strontium Titanate

Two series of experiments were performed to study the experimental conditions for the formation of {111} twins and related microstructures in barium strontium titanate ((Ba, Sr)TiO₃). In the first series, the phase equilibria in the BaTiO₃–SrTiO₃–TiO₂ system were determined. XRD and WDS analysis, done in the BaTiO₃-rich region, of 45(Ba,Sr)TiO₃–10TiO₂ samples annealed at 1250°C for 200 h in air showed that (Ba,Sr)TiO₃ was in equilibrium with Ba₆Ti₁₇O₄₀ (B₆T₁₇) and Ba₄Ti₁₃O₃₀ phases with strontium solubility (Sr/(Ba + Sr)) of ∼0.02 and 0.20, respectively. In the second series the microstructures of samples consisting of a mixture of (Ba,Sr)TiO₃ and 2.0 mol% TiO₂, were observed after sintering at 1250°C for 100 h in air. {111} twins formed only in the samples with faceted B₆T₁₇ second phase particles, similar to the case of BaTiO₃. In these samples, abnormal grain growth occurred in the presence of the {111} twins. In contrast, no {111} twins formed and no abnormal grain growth occurred in the samples containing second phase particles other than B₆T₁₇. With an increased substitution of strontium for barium, the aspect ratio of abnormal grains containing {111} twin lamellae was reduced. This result was attributed to a reduction in the relative stability of the {111} planes with the strontium substitution.     

Necessary Conditions for the Formation of {111} Twins in Barium Titanate

The experimental conditions for {111} twin formation in BaTiO₃ were investigated. When BaTiO₃ compacts without excess TiO₂ were sintered either in an oxidizing atmosphere (air) or in a reducing atmosphere (95N₂–5H₂), no {111} twins formed within the BaTiO₃ grains and no abnormal grain growth occurred. In contrast, many {111} twins were present within the abnormally grown grains in the excess-TiO₂-containing BaTiO₃ samples sintered in air, while no twins were observed in the excess-TiO₂-containing samples sintered in 95N₂–5H₂. X-ray diffraction analysis showed that excess TiO₂ forms a Ba₆Ti₁₇O₄₀ phase during sintering with the space group A 2/ a in air and a Ba₆Ti₁₇O_{40− x} phase with the space group C in 95N₂–5H₂. It appears therefore that excess TiO₂ and an oxidizing atmosphere are necessary for {111} twin formation in BaTiO₃. These results may also indicate that the interface structure between BaTiO₃ and Ba₆Ti₁₇O₄₀ influences the twin formation.     

Phase Equilibria in the TiO2-Rich Region of the System BaO-TiO2

Phase relations in the system BaO-TiO₂ from 67 to 100 mol% TiO₂ were investigated at 1200° to 1450°C in O₂. Data were obtained by microstructural, X-ray, and thermal analyses. The existence of the stable compounds Ba₆Ti₁₇O₄₀, Ba₄Ti₁₃O₃₀, BaTi₄O₉, and Ba₂Ti₉O₂₀ was confirmed. The compound BaTi₂O₅ is unstable and either forms as a reaction intermediate below the solidus or crystallizes from the melt. The compounds Ba₆Ti₁₇O₄₀ and Ba₄Ti₁₃O₃₀ decompose in peritectic reactions, and BaTiO₃ and Ba₆Ti₁₇O₄₀ react to form a eutectic. Special conditions are required for the formation of Ba₂Ti₉O₂₀, which decomposes in a peritectoid reaction at 1420°C. The new phase diagram is presented.     

Synthesis and Characterization of Barium Lanthanum Titanates

Ternary compounds in the system BaO—TiO₂—La₂O₃ were prepared by the solid-state reaction technique at temperatures between 1300° and 1400°C using precursor oxides as the starting materials. In an alternative processing technique, BaTiO₃ was reacted with appropriate proportions of prefabricated lanthanum titanates at 1350°C to obtain the compounds. Two compounds were identified in the TiO₂-rich region of the system. The X-ray powder diffraction pattern of a compound with a chemical composition BaLa₂Ti₃O₁₀ (BaO·La₂O₃·3TiO₂) is indexed on the basis of an orthorhombic unit cell with a = 7.665 × 10⁻¹ nm, b = 28.524 × 10⁻¹ nm, and c = 3.876 × 10⁻¹ nm. The other compound, which has a chemical composition Ba₄La₈Ti₁₇O₅₀ (BaO·La₂O₃·4.25TiO₂) occurs in a narrow homogeneity range within the system. The X-ray powder diffraction pattern of the compound is indexed on the basis of an orthorhombic unit cell with a = 12.317 × 10⁻¹ nm, b = 22.394 × 10⁻¹ nm, and c = 3.881 × 10⁻¹ nm. Both the compounds are compatible with BaTiO₃ and form pseudobinary joins with BaTiO₃ in the system BaO—TiO₂—La₂O₃.     

Effects of B2O3 on the Phase Stability of Ba2Ti9O20 Microwave Ceramic

Preparation of dense and phase-pure Ba₂Ti₉O₂₀ is generally difficult using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba₂Ti₉O₂₀ equilibrium phase boundary in the BaO–TiO₂ system at high temperatures. In this study, the effects of B₂O₃ on the densification, microstructural evolution, and phase stability of Ba₂Ti₉O₂₀ were investigated. It was found that the densification of Ba₂Ti₉O₂₀ sintered with B₂O₃ was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid-state sintering became dominant because of the evaporation of B₂O₃. With the addition of 5 wt% B₂O₃, the ceramic yielded a pure Ba₂Ti₉O₂₀ phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO₂ or ZrO₂. The facilities of B₂O₃ addition to the stability of Ba₂Ti₉O₂₀ are apparently due to the eutectic liquid phase which accelerates the migration of reactant species.     

Genesis of the (111) Twin in Barium Titanate

On the basis of the topotaxy between BaTiO₃ and Ba₆Ti₁₇O₄₀ found recently, a model of a nonconservative (111) twin in TiO₂-rich BaTiO₃ was constructed. The model consists of several (001) layers of Ba₆Ti₁₇O₄₀ intergrown between (111) layers of BaTiO₃, the core of the twin being a slightly modified double layer of Ba₆Ti₁₇O₄₀ containing face-sharing octahedra. Using this model, anomalous grain growth below the eutectic temperature and preferential growth of (111) twins in a reducing atmosphere were explained, as well a nucleation of butterfly twins.     

Comprehensive Linkage of Defect and Phase Equilibria through Ferroelectric Transition Behavior in BaTiO3-Based Dielectrics: Part 1. Defect Energies Under Ambient Air Conditions

Defect and phase equilibria have been investigated via the ferroelectric phase transition behavior of pure and equilibrated nonstoichiometric BaTiO₃ powder samples. Through fabricating the BaTiO₃ materials under highly controlled conditions to preserve the equilibrium conditions with respect to Ba/Ti ratio, annealing temperature ( T ), and oxygen partial pressure ( P _O2), systematic variations in the phase transition temperature can be noted with respect to Ba/Ti ratio and T . From the data extracted, we can then determine solubility limits. Equilibrating the defect reactions at the solubility limits provides a direct approach to identify and calculate the defect energetics. The phase transition temperature decreased with increasing concentration of the TiO₂ partial-Schottky defects (BaTi_1−δO_3−2δ) and the BaO partial-Schottky defects (Ba_1−δTiO_3−δ), and showed discontinuous changes in the two-phase region. The formation enthalpy and entropy for the partial-Schottky defect reactions was evaluated to be 2.32±0.1 eV and 10.15±0.7 k _B for the BaO partial-Schottky defect, and 2.89±0.1 eV and 8.0±1.5 k _B for the TiO₂ partial-Schottky defects equilibrated under air annealing conditions.     

Tunable Microwave Properties of Barium Titanate-Based Ferroelectric Glass-Ceramics

BaTiO₃-based ferroelectric glass-ceramics with the composition 0.65(Ba_{1− X} Sr _X )TiO₃·0.27SiO₂·0.08Al₂O₃ ( X = 0.2–0.6) were fabricated, and their tunable dielectric properties were measured at microwave frequency. Major crystalline phases that precipitated during thermal treatment up to 1000°C were (Ba,Sr)TiO₃, Ba₂TiSi₂O₈, and BaAl₂Si₂O₈. The Curie temperatures of the heated bulk samples were successfully tuned near room temperature at the composition between X = 0.2 and 0.3. A thick-film sample with X = 0.3 showed 27% tunability at 5 GHz under 10 kV/cm bias voltage.     

Effects of Ba6Ti17O40 on the Dielectric Properties of Nb-Doped BaTiO3 Ceramics

The dielectric properties, including the DC breakdown strength, of 1 mol% Nb⁵⁺-doped BaTiO₃ ceramics with different quantities of excess TiO₂ have been investigated. The breakdown strength was found to decrease with increasing TiO₂ content, but could not be readily explained by relative density and grain size effects. The decrease in the breakdown strength from a stoichiometric BaTiO₃ composition to samples with excess TiO₂ is believed to be due to the field enhancement effect (up to a factor of 1.40) at the BaTiO₃ matrix because of the presence of a Ba₆Ti₁₇O₄₀ second phase. The thermal expansion coefficient mismatch between the BaTiO₃ matrix phase and the Ba₆Ti₁₇O₄₀ phase may also result in a low breakdown strength. The dielectric properties of the pure Ba₆Ti₁₇O₄₀ phase were also investigated and are reported herein.     

Incorporation of Yttrium in Barium Titanate Ceramics

In this work, X-ray diffractometry and scanning electron microscopy techniques were used to study the incorporation of yttrium in BaTiO₃, with the following nominal compositions: (Ba_{1- x} Y _x )TiO₃(0.015 ≤ to x ≤ to 0.08), (Ba_{1- x} Y _x )Ti_{1- x /4}-(□_Ti) _{x /4}O₃(0.005 ≤ to x ≤ to 0.1), and Ba(Ti_{1- y} Y _y )O_3-delta(0.028 ≤ to y ≤ to 0.258). The phase assemblage and the lattice parameters indicated a slight solubility (∼1.5 at.%) of yttrium at the Ba sites at 1440°C but a high solubility (∼12.2 at.%) of yttrium at the Ti sites at 1515°C. When BaTiO₃was heavily doped with yttrium at the Ti sites (a yttrium concentration ( y ) of <0.059), the crystallographic structure was tetragonal, whereas for y greater than equal to 0.059, the crystallographic structure was cubic.     

Metal Organic Resin Derived Barium Titanate: I, Formation of Barium Titanium Oxycarbonate Intermediate

Crystallization of BaTiO₃ from an X-ray amorphous, metal organic precursor was investigated by comparing samples heated in O₂, air, argon, and CO₂. It is evident that an intermediate barium titanium oxycarbonate phase forms between 500° and 620°C and that BaTiO₃ forms directly by the endothermic decomposition of this phase between 635° and 700°C. From thermodynamic calculations, thermal analysis, X-ray diffraction, and Raman spectroscopy, it is concluded that the intermediate oxycarbonate is a highly disordered, metastable, and weakly crystalline phase with a stoichiometry close to Ba₂Ti₂O₅CO₃.     

Orientation Relationships of Reactively Grown Ba6Ti17O40 and Ba2TiSi2O8 on BaTiO3 (001) Determined by X-ray Diffractometry

Fresnoite grows at 700° and 800°C, and Ba₆Ti₇O₄₀ grows at 1200°C with definite orientations, which are determined by X-ray diffraction pole figure analysis. Partially textured fresnoite is formed at higher temperatures. The SiO₂ films react with the BaTiO₃ crystals, forming the phases Ba₂TiSi₂O₈ (fresnoite) and Ba₆Ti₁₇O₄₀. At 700° and 800°C, both phases grow with definite orientations, which are determined by X-ray diffraction pole figure analysis. Partially textured polycrystalline phases are formed at higher temperatures.     

Microstructural Changes during the Reduction/Reoxidation Process in Donor-Doped BaTiO3 Ceramics

Microstructural changes occurring during oxidation of the reduced form of donor-doped BaTiO₃ (Ba_{1− X} D _X ^.Ti_{1− X} ⁴⁺Ti _X ³⁺O₃) and during reduction of the oxidized form of donor-doped BaTiO₃ (Ba_{1− X} D _X ^.Ti_{1− X /4}⁴⁺( V _Ti^) _{X /4}O₃) were studied using TEM. Samples of both types of solid solutions, containing different La concentrations (from 2 to 20 mol% La), were prepared by sintering under reducing conditions and in air, respectively. The reduced form of donor-doped BaTiO₃ was oxidized by annealing at high temperatures (1150° and 1350°C) in air, while the oxidized form was reduced by annealing under reducing conditions. Because of oxidation of the reduced phase of donor-doped BaTiO₃, the Ti-rich phases Ba₆Ti₁₇O₄₀ and BaLa₂Ti₄O₁₂ were precipitated. Reduction of the oxidized form caused precipitation of the Ba-rich phase Ba₂TiO₄ preferentially inside the matrix grains. All precipitates had well-defined orientational relationships with the perovskite matrix.     

Effect of pH on the Chemistry of the Barium Titanium Citrate Gel and Its Thermal Decomposition Behavior

In this investigation, X-ray diffractometry and ¹³C NMR spectroscopy were used to characterize the effect of pH on the formation of gel and its thermal decomposition by the Pechini process. It was found that the major effect of pH was to destroy the esterification between citric acid (CA) and ethylene glycol, which in turn could influence Ba species and the formation of a mixed-metal CA complex. It was also found that barium carbonate was derived from the Ba species and was not related to the instability of Ba₂Ti₂O₅CO₃, but Ba₂Ti₂O₅CO₃ was formed by the thermal decomposition of a mixed-metal CA complex. Moreover, BaTiO₃ formed via two routes, the reaction of BaCO₃ with Ti species and thermal decomposition of Ba₂Ti₂O₅CO₃.     

Phase Equilibria in the System BaO–TiO2

The system BaO-TiO₂ was investigated using quenching, strip-furnace, and thermal techniques. Five compounds were found to exist in the system: Ba₂TiO₄, BaTiO₃, BaTi₂O₅, BaTi₃O₇, and BaTi₄O₉. Of these, only barium metatitanate (BaTiO₃) melts congruently (at 1618°C.). The dititanate melts incongruently at 1322° C. to yield BaTiO₃ and liquid; the trititanate melts at 1357°C. to yield BaTi₄O₉ and liquid; the tetra-titanate melts to TiO₂ and liquid at 1428° C. The nature of melting of the orthotitanate could not be determined accurately because of the high temperature involved and the rapid reaction with platinum. The two eutectics in the system occur between Ba₂TiO₄ and BaTiO₃ at 1563°C. and between BaTi₂O₅ and BaTi₃O₇ at 1317°C. The temperature of the cubic-hexagonal transition in barium metatitanate was determined as 1460°C. and the transition has been shown to be reversible. The transition temperature is raised sharply by the addition of a small percentage of TiO₂ although the extent of solid solution is quite limited. Some applications to the manufacture of titanate bodies and to the growth of single crystals of barium metatitanate are discussed.     

Texture Development in Barium Titanate and PMN–PT Using Hexabarium 17-Titanate Heterotemplates

Bulk BaTiO₃ ceramics with 〈111〉-texture have been prepared by the modified templated grain growth method, using platelike Ba₆Ti₁₇O₄₀ particles as templates, and the mechanism of texture development is examined. The Ba₆Ti₁₇O₄₀ particles induce the abnormal growth of BaTiO₃ grains, and a structure similarity between {001} of Ba₆Ti₁₇O₄₀ and {111} of BaTiO₃ gives 〈111〉-texture to abnormally grown BaTiO₃ grains. Thus, the 〈111〉-texture develops in the BaTiO₃ matrix. The use of platelike Ba₆Ti₁₇O₄₀ particles has been extended to a 0.65Pb(Mg_1/3Nb_2/3)O₃–0.35PbTiO₃ matrix, but the matrix phase is decomposed by extensive chemical reactions between the matrix and template phases.     

Intermediate Phase Development in Phosphorus-Doped Barium Titanate

In the present work, the phase formation and thermal evolution in phosphorus-doped BaTiO₃ have been studied using differential thermal analysis, X-ray diffractometry, scanning electron microscopy coupled with energy-dispersive spectroscopy, transmission electron microscopy, and high-temperature nuclear magnetic resonance. Phosphorus cations that are incorporated from ester phosphate form a surface layer that covers the BaTiO₃ particles. This layer acts as a reactive coating during sintering. Phosphorus-doped BaTiO₃ samples that have been treated at temperatures of 650°–900°C show the presence of crystalline Ba₂TiP₂O₉ and/or Ba₃(PO₄)₂ phases. The appearance of secondary phases is dependent on the cooling rate. Higher temperatures (900°–1200°C) result in the presence of a phosphorus–BaO-rich phase that covers the BaTiO₃ particles. As a consequence, the remaining titanium-rich BaTiO₃ drives the formation of a liquid phase at temperatures >1200°C. In regard to the reported sintering behavior of P⁵⁺-doped BaTiO₃, the formation of a phosphorus–BaO-rich phase that covers the BaTiO₃ particles could be the origin of the improved porosity coalescence and removal that is observed at the earlier stages of sintering.     

Solid-State Preparation of BaTiO3-Based Dielectrics, Using Ultrafine Raw Materials

BaTiO₃ and Ba(Ti,Zr)O₃ dielectric powders have been prepared from submicrometer BaCO₃, TiO₂, and ZrO₂. By use of submicrometer BaCO₃ the intermediate formation of Ba₂TiO₄ second phase can be widely suppressed. Monophase perovskites of BaTiO₃ were already formed at 900°C and Ba(Ti,Zr)O₃ at 1050°C. Aggregates of very small subgrains could be easily disintegrated to particle sizes <0.5 μm.     

Temperature-Dependent Phase Boundaries for BaTi409, Ba4Ti13030, and Ba6Ti17040

The phase boundaries of BaTi₄O₉ and Ba₄Ti₁₃O₄₀ are temperature dependent and curve toward BaTiO₃ above 1250°C. The appearance of a barium-rich surface phase during slow cooling is a sensitive indicator of a temperature-dependent boundary. For both compounds the surface phase which is barium-rich and has u strong X-ray peak at d=0.232 nm. No surface phase was detected on Ba₆Ti₁₇O₄₀; therefore, its phase boundary is independent of temperature.